The present invention relates to packaging film. Specifically, the present invention relates to multiple layer heat sealable nonoriented packaging films exhibiting excellent moisture barrier properties.
Biaxially oriented polyethylene terephtalate, commonly referred to as "oriented polyester" or "OPET," is a common packaging film with over 200 million lbs. used annually.
An important criteria for any packaging film is its ability to be heat sealed to itself as well as to other substrates. OPET films are not heat sealable. In recent years, however, heat sealable OPET has become commercially available incorporating amorphous homopolymers or copolymers of PET as heat sealants applied either as a coating or coextrusion. OL Mylar is an example of a heat sealable OPET film having the heat sealants applied as a coating. OL Mylar is a registered trademark of E. I. DuPont de Nemours and Company. Melinex 850 is an example of a heat sealable OPET film having heat sealants applied as a coextrusion. Melinex is a registered trademark of Imperial Chemical Industries, Ltd.
Recently, heat sealable OPET films have been used as the inside layer of a paper-film laminated bag for cooking popcorn and other foodstuffs in a microwave oven. Microwave popcorn bags are described in U.S. Pat. Nos. 3,973,045 and 4,450,180. However, the bags of these inventions do not contain an inside film liner, rather they consist of two plys of paper.
Typically, the outer ply is a bleached Kraft paper, while the inner ply is a greaseproof paper such as glassine. These bags are intended for a frozen product in which the bag is filled with the popcorn-cooking oil mixture and then frozen. The product is shipped and stored frozen.
In recent years, "shelf stable" products such as microwave popcorn bags have been introduced. These products do not require any refrigeration. However, the greaseproof paper lined bag used for the frozen product was not adequate for the shelf stable product in that the cooking oil in the unfrozen state would leak through and stain the outer paper liner. This problem was eliminated by changing the inner ply to a plastic film. In particular, heat sealable OPET films are used as the inner ply of the shelf stable bag. An example of this product is Orville Redenbacher's Gourmet Microwave Popping Corn. Orville Redenbacher's and Gourmet are registered trademarks of the Beatrice Companies, Inc.
OPET films are manufactured by an orientation process utilizing tenter frame or "double bubble" techniques. Tentering of plastic films is illustrated in U.S. Pat. No. 2,823,421 and double-bubble orientation is illustrated in U.S. Pat. No. 3,555,604. U.S. Pat. Nos. 4,141,736 and 4,207,363 describe oriented films made using the tenter and double-bubble processes, respectively. The orientation process comprises the steps of (1) extruding a thick web and cooling to a solid state; (2) precise reheating of film to the "orientation" temperature, (3) stretching the film in both longitudinal and transverse directions, and (4) reheating film to relieve internal stresses. Melinex 850 comprises two layers coextruded in step (1) of the orientation process prior to reheating to the "orientation" temperature. An example of a coextruded OPET is disclosed in U.S. Pat. No. 4,375,494. This heat sealable OPET differs from the films of this invention in that the film is oriented.
Alternatively, monolayer OPET can be made heat sealable by laminating it to a heat sealant film. This lamination is done in a separate manufacturing operation. Typically the heat sealant is an olefinic film such as polyethylene. An example of such a lamination is disclosed in U.S. Pat. No. 4,178,401.
An alternative to utilizing heat sealable OPET films for this popcorn application is the use of films from coextrusions of polycarbonate, polyester and/or copolyesters. Such films are described in copending U.S. patent application Ser. No. 759,392. Similar films coextruded without polycarbonate are described in copending U.S. Patent Application Ser. No. 759,391.
A well known problem with the "shelf stable" microwave popcorn bag is moisture loss from the popcorn as the product ages. The heat sealable OPET films used in this product do not provide a sufficient barrier to the loss of moisture. The moisture content of unpopped popcorn significantly affects the size of the kernel after popping. The higher the moisture content of the unpopped kernel, the larger the size of the popped kernel. In other words, higher moisture content corn will yield "fluffier" popped corn.
The loss of moisture from corn stored in microwave popcorn bags made with heat sealable OPET films results in smaller popped kernels. This translates to a lower volume of popped corn with inferior (compared to fresh corn) texture, i.e., small, hard and dry popped kernels. The loss of volume and degradation of popped corn texture with product age is objectionable to the producer as well as the end user.
One approach used to improve the moisture barrier of the microwave popcorn bag has been to coat the heat sealable OPET with polyvinylidene chloride (PVDC). The use of PVDC coated film has been shown to significantly reduce corn moisture loss hence reduce pop volume loss and popcorn texture degradation due to aging. However, there are two major objections to the use of the PVDC coated films: (1) since the coating requires an additional material and manufacturing step, the finished film is more expensive than non-coated OPET; and (2) the PVDC absorbs microwave energy which, in turn, causes the bag to get very hot. The excessive heat generated can cause charring of the outer paper layer and the degradation of the PVDC which can, in turn, produce noxious odors.
Another approach to modifying heat sealable OPET films to improve their moisture barrier has been to laminate an olefinic film, such as polypropylene, to the OPET film. The olefinic film provides improved moisture barrier but the lamination requires two additional manufacturing steps: (1) production of the olefinic film, and (2) lamination of this film to the OPET film.
Multiple layer nonoriented moisture barrier heat sealable films containing no PVDC are well known. For example, high density polyethylene (HDPE) is coextruded with low density polyethylene homopolymer (LDPE) or ethylene vinyl acetate copolymer (EVA) for applications such as cereal box liners. The HDPE provides the required moisture barrier properties while the LDPE or EVA provide heat sealability.
Coextrusions comprising polyethylene and a heat seal layer are well known. The most common are two layer ("AB") coextrusions comprising a polyethylene base layer ("A") and a heat seal layer ("B").
U.S. Pat. No. 4,390,573 describes a coextrusion of a resinous blend of high density polyethylene and a linear low density ethylene-alpha-olefin copolymer. The film is made by blown coextrusion and is intended for the manufacture of improved trash bags. This film is representative of polyethylene coextrusion.
U.S. Pat. No. 4,188,441 describes a two layer ("AB") coextrusion of a base layer comprising high density polyethylene and a heat layer comprising of a blend of an ethylene copolymer and a ionomer copolymer.
The films of the above-referenced patents differ from the films of the present invention in that they do not utilize a polyester/copolyester as the heat sealant layer. Additionally, the above-referenced films are simple two layer structures wherein a tie layer is not required when coextruding polymers of similar chemistry.
U.S. Pat. No. 4,477,532 describes the use of tie layers in coextruded films. Specifically, a tie layer comprising blends of a nonelastomeric graft copolymer of a nonpolar ethylene homopolymer or copolymer backbone grafted with at least one grafting monomer comprising one or more of polymerizable ethylenically unsaturated carboxylic acids or the anhydrides of such acids blended with an elastomer-free binding resin that is a mixture of one or more linear low density polyethlenes and one or more polypropylenes is disclosed. Composite structures of the tie layer and polar polymers, polypropylene, solid metals, glass, paper, wood or cellophane are disclosed. Three layer structures are disclosed.
U.S. Pat. No. 4,188,443 discloses oriented shrink films. The preferred shrink film has five layers in which the middle layer is a polyester or copolyester, the two layers on either side of the middle layers are ethylene-vinyl acetate copolymers or terpolymers and two outer layers, one on each side of the film, are polypropylene. A three layer film having a middle layer of ethylene-vinyl acetate copolymer or terpolymer, an outer layer of polyester or copolyester and an outer layer of polyethylene is disclosed. Specifically, the terpolymer PLEXAR 3 is disclosed. PLEXAR is a registered trademark of Norchem, Inc. U.S. Pat. No. 4,188,443 is in the art of oriented shrink films and does not teach the use of such film or any layers thereof in nonoriented heat sealable film applications. Furthermore, U.S. Pat. No. 4,188,443 specifically utilizes polyethylene homopolymer or copolymers as the heat sealant rather than the polyester/copolyester layer.
U.S. Pat. No. 3,925,591 discloses nonoriented coextrusions of polyethylene, ethylene vinyl acetate copolymer (EVA) and polyester/copolyesters. The film is intended for packaging applications where the polyethylene layer serves as a heat sealant. The bond between the EVA and polyester, copolyester would be weak due to incompatibility of the two polymers. The present invention provides superior layer bond strength by utilization of a tie layer to bond the polyester/copolyester to the polyethylene base layer. Furthermore, U.S. Pat. No. 3,925,591 does not teach the use of polyester/copolyester as the heat sealant layer. Rather, it refers to laminates of this field which typically use polyolefins as the heat sealant.
The film of the present invention is unique in that the polyester/copolyester layer is utilized as the heat sealant layer. Typically, laminations and coextrusions of this type utilize polyethylene homopolymers or copolymers as the heat sealant layer. The present invention discloses surprising results when using polyester/copolyester as the heat sealant. In particular, it has been found that the film of the present invention can be utilized in extremely high temperature demanding applications and in providing differential seal properties. Furthermore, the preferred embodiment discloses a specific copolyester which provides heat seal properties heretofor unobtainable with polyethylenes or other polyesters/copolyesters. Specifically, these properties include a broad heat seal range to a wide variety of substrates and the ability to withstand extremely high temperatures up to 500.degree. F. Despite the fact that the film of the present invention may contain up to 90% w/w of olefins (base layer and tie layer), which melt at 250.degree. F. or less, suprisingly such film can be used in high temperature applications such as microwave popcorn bags where temperatures can exceed 500.degree. F.
The films of the present invention provide an excellent barrier to moisture loss. When used as the inner liner of a microwave popcorn bag, the loss of moisture from the corn is significantly reduced compared to bags lined with heat sealable OPET. As a result, corn stored in bags lined with the films of this invention does not exhibit any significant loss of popped volume or texture with aging.
The nonoriented, heat sealable moisture barrier films of the present invention do not contain any PVDC, do not become excessively hot or degrade when used in microwave cooking applications such as a liner of a popcorn bag for microwave ovens, and can be produced on conventional film extrusion equipment in one manufacturing step. Unlike PVDC coated OPET film or laminations of polyolefin films to OPET, the films of this invention can be made in a single manufacturing step. Furthermore, the film costs about the same or less than the non-PVDC coated OPET films. Therefore, a moisture barrier popcorn bag can be made with films of this invention without an increase in cost over the typical bag made with OPET.
The films of the present invention comprise a base layer ("A"), of polyethylene, preferably high density, a tie layer ("B") of modified polyethylene homopolymer, modified polyethylene copolymer, unmodified polyethylene copolymer or combinations thereof, and a heat seal surface layer ("C") of polyester, copolyester or blends thereof yielding a three layer film ("ABC") or a five layer film ("CBABC") which are preferred embodiments. The films can be produced by conventional blown or cast coextrusion techniques.
Conventional blown film coextrusion techniques and equipment therefor are known in the art and are commercially available. Also, conventional cast film coextrusion techniques and equipment therefore is known in the art and is commercially available. The following U.S. patents disclose various extrusion techniques and equipment therefor: 4,484,883; 4,483,812; 4,465,449; 4,405,547; 4,403,934; 3,611,492; 3,559,239; 3,476,627; 3,337,914; 3,223,761 and 3,467,565.
Also, two conventional cast coextrusion techniques are known in the art. The first method combines the molten polymers in a combining adaptor prior to entering the slot cast die. The second method does not bring the molten polymers in contact with each other until the polymer melt streams are inside the die. Either method will yield a cast coextruded film with very similar properties.
The heat sealant layer ("C") consists of a polyester or copolyester polymer or blend thereof of compatible polymers. The ("C") layer promotes a heat seal when the film is heat sealed to (1) the ("C") layer of the film, or (2) other films or rigid substrates. The heat seal is accomplished by applying heat and pressure against the film and substrate. An example of a heat seal is a seal found on packages of snack foods or the bottom of a plastic trash bag. The heat seal can be either peelable or fused. Heat seals made at the low end of the film's heat seal range (temperature) are peelable while seals made at the upper end of the range are fused.
Optionally, the films can be made with a ("CBABC") configuration where the base material ("A") is "sandwiched" between two pairs of tie and heat sealant layers. This structure provides a film which is heat sealable on both sides, which is preferred in some packaging applications.
The present invention provides compositions of multiple layer moisture barrier heat sealable films which can be used in high temperature applications such as the microwave popcorn bag.
The present invention demonstrates the following advantages over the films disclosed in the prior art: (1) the films of the present invention contain a high temperature polymer (polyester/copolyester) as the heat sealant allowing the film to be used in high temperature applications, including but not limited to microwave popcorn bags; (2) the films of the present invention exhibit a very broad heat seal range; (3) the films of the present invention heat seal to a wide variety of substrates; and (4) the films of the present invention provide differential seal properties for peelable or fusion seals.
It is the object of the present invention to provide film compositions which can be used in place of OPET films, PVDC coated OPET films or olefin-OPET laminated films in extremely demanding applications such as microwave popcorn bags or the like.
It is another object of the present invention to provide multiple layer heat sealable films manufactured by conventional cast or blown film nonorienting coextrusion techniques which can be utilized as an inside liner of a laminate bag such as a microwave popcorn bag or the like produced by laminating the film as the inside liner to a paper layer and forming the bag from the resulting laminate.
It is still another object of this invention to provide films which can be used to produce microwave popcorn bags with superior moisture barrier properties and to cost relatively the same or less than conventional bags made with heat sealable OPET films.
These and other objects of the present invention will be apparent from the description of the preferred embodiments which follows. Such objects are not intended to limit the scope of the present invention.